Estimation of mass transfer through a hemodialyzer: theoretical approach and clinical applications

The estimation of the solute mass transfer through a dialyzer is generally based on the solute dialysance, but the concept of dialysance has been precisely defined only in the case of a merely diffusive transfer. In actuality the mass transfer of a solute is also influenced by the ultrafiltration responsible for a convective transfer and, if the solute is an ionic substance, by the transmembrane gradient of electrical potential due to the Gibbs-Donnan effect. The aim of this paper is to generalize the concept of dialysance when the diffusive, convective, and electric components of the transfer are simultaneously active. There are at least 3 modes to generalize the concept of dialysance for it to be identical, when the amount of ultrafiltration and the Gibbs-Donnan effect are negligible, to the usual dialysance defined in the case of a merely diffusive transfer. The dialysance can be defined so that it can be equal to the clearance for a solute absent from the dialysate again, so that it still represents the rate at which the plasma concentration of a given solute is reaching its equilibrium value, or so that it represents the merely diffusive component (independent of the ultrafiltration rate) of the mass transfer. This generalized concept of dialysance can be useful to provide a real-time estimation of the effective dialysis dose actually delivered to the patient; to automatically optimize, by a biofeedback process, the sodium balance during a hemodialysis session; and to adapt dialysate concentrations for new hemodialysis techniques with convective transfer (acetate-free biofiltration).